function res = fn_ff2(k, kc)

res = 2/pi/kc/(4*k^2+1) ...
    +4/pi*(4*k^2-1)/(4*k^2+1)^2*(pi/2+atan(2*(k-kc))) ...
    +8*k/pi/(4*k^2+1)^2*log((1+4*(k-kc)^2)/(4*kc^2));

end